UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Non-working Commodore PET 3016

SiriusHardware · 12th Feb 2021, 2:10 pm

OK Colin, let's hope he has something for you there.

Information for Owen: The original Mask-Programmed PROMS used in the PET have -up to- three CS pins, named CS1, CS2, CS3. The exact function and 'sense' of these pins was programmed at the manufacturing stage, and the ones in the PET are set up like this:

CS1 (Pin 20) - Active low chip select
CS3 (Pin 21) - Active high chip select

CS2 (Pin 18) - programmed as address line A11. That is why, when you look at the PET circuit diagram, the PROMs all have a CS1 and CS3 but mysteriously, no CS2.

In the PET, pin 21 (active high chip select) is held at permanent +5V which is fortunate, as that pin coincides with VPP which also needs to be held at +5V for read mode on on 2532, etc EPROMs.

12th Feb 2021, 2:10 pm	#585
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,586	Re: Non-working Commodore PET 3016 OK Colin, let's hope he has something for you there. Information for Owen: The original Mask-Programmed PROMS used in the PET have -up to- three CS pins, named CS1, CS2, CS3. The exact function and 'sense' of these pins was programmed at the manufacturing stage, and the ones in the PET are set up like this: CS1 (Pin 20) - Active low chip select CS3 (Pin 21) - Active high chip select CS2 (Pin 18) - programmed as address line A11. That is why, when you look at the PET circuit diagram, the PROMs all have a CS1 and CS3 but mysteriously, no CS2. In the PET, pin 21 (active high chip select) is held at permanent +5V which is fortunate, as that pin coincides with VPP which also needs to be held at +5V for read mode on on 2532, etc EPROMs.